Gas mixtures containing nitrous oxide



United States Patent 29,417/ 61 15 Claims. (Cl. 16752 This invention relates to gas mixtures containing nitrous oxide.

3,192,195 7 Patented June 29, 1965 ice for the permanent" gases to be distributed in cylinders at a pressure of 132 atmospheres or higher. Atmospheres above 300 are not practical, and are not contemplated by Nitrous oxide is used on a considerable scale as an anaesthetic gas and is normally administered in admixture with oxygen. For this purpose the two gases are stored in separate pressure containers, usuallytermed cylinders, the oxygen being contained as a high pressure gas and the nitrous oxide as a liquefied gas at a lower pressure. The required mixture is commonly prepared at the time of use by controlled discharge of the respective gases. Conventionally, the control means include a pressure regulator associated with each cylinder and some kind of proportioning device which meters the flow of the gases to a mixing chamber. v I

Oxygen is one of the so-called permanent gases, that is to say, it remains in the gas phase at ambient temperatures whatever the pressure. The pressure in an oxygen cylinder progressively falls ,as the contents are used and does not vary much with the ambient temperature. A pressure gauge can therefore, serve as a simple contents indicator.

On the other hand, nitrous oxide can be and usually the present invention. The cylinders are designed for such duty and it is uneconomic to use them for the storage and transport of gases at considerably lower pressures;

Contrary to expectations, it has now been found possible to store -a gaseous mixture containing up to about 75% nitrous oxide at a partial pressure of nitrous oxide which can be substantially in excess of 50 ats. without any liquefaction taking place.

It is thus possible to conserve a mixture of nitrous oxide with a permanent gas such as oxygen in a cylinder at filling pressures normally used for the permanent gases and to withdraw from such cylinder a mixture which remains homogeneous at all demand rates. Storage and dispensing apparatus is thereby simplified and the need for a proportioning device avoided.

According to this invention, a method of forming ahomogeneous mixture of a permanenfgas, and ,up to 75% of nitrous oxide comprises charging the gases in the predetermined required proportions into a pressure container until a pressure is'reached at which the partial is stored under pressure in the liquid phase. Under static 7 effect on pressure. No simple means for determining the contents of a nitrous oxide cylinder under normal conditions of use is available and there is therefore a danger that the supply of nitrous oxide will-run out unexpectedly.

The control of the two gases to give a mixture of constant predetermined composition is also a difiicult matter and requires somewhat elaborate proportioning mechanism constructed to closetolerances. Departure from the pre-set tolerances areliable to occur in use resulting in variations from the indicated compositionf Regulartesting and servicing are therefore essential where such mechanisms are concerned. I It is known that gaseous mixtures contain-ing nitrous oxidecan exist in a homogeneous state at pressures up to a partial pressure of nitrous oxide of atmospheres. At higher partial pressures, liquefaction of part of the nitrous oxide would be expected so that the mixture would no longer be homogeneous. Thus for a mixture containing 70% of nitrous oxide and 30% of oxygen one would expect the upper limit for a homogeneous mixture to be about 72 atmospheres. It is, however, standard practice pressure of the nitrous pheres.

Preferably the mixture iscompressed to at. least 132 atmospheres. f 'Acording to another aspect of this invention there is provided -a homogeneous gaseous medium comprising up to 75% nitrous oxide in admixture with a permanent 'gas confined in a container at a pressure such that the oxide is greater than.50.atmospartial pressure of nitrous oxide is greater than 50 atmospheres.

The permanent gas most commonly used with nitrous oxide is oxygen. For inducing a state of analgesia, a nitrous oxide/ oxygen mixture containing nitrous oxide in the range of 50 to 60% is frequently administered. The provision of the gases in premixed form at high pressure, for example, 132 ats., eliminates the need to use gas mixing equipment at the time of administration. This has especial advantage in reducing the impedimenta of the travelling midwife attending domiciliary confinements.

For inducing a state of anaesthesia, a more potent drug such as halothane or cyclopropane may be added as a constituent of the nitrous oxide/oxygen mixture. Up to about 1% halothane, volume by volume, or up to 5%v cyclopropane may be included in the nitrous oxide/ oxygen mixture as the potent adjuvant, while maintaining the contents of the pressure container entirely in-the gaseous phase. When cyclopropane is the potent adjuvant, the mixture may be inflammable and the usual precautions should be taken to guard against risk of static discharge.

In formulating the mixture in accordance with'the invention, the required proportion of nitrous oxide is preferably charged into a valved pressure container first, the quantity being determined either by weight or by volume. The required'proportionof permanent gas then added, either continuously or intermittently so as to allow equilibrium to take place. The attainment of equilibrium, namely the complete disappearance of the liquid phase, may be accelerated by agitating the contents. For example, the container may be shaken during charging with oxygen,

or the container may be rolled about after charging.

Alternatively the container may be inverted so that the. oxygen bubbles through the liquid nitrous oxide.

In an alternative method of formulating the mixture, the nitrous oxide and oxygen are charged simultaneously into the container. The two gases are preferably premixed, either in a gas holder from which they are pumped into the container, or continuously by the method known as streaming in in which the flow rates of the separate gas streams are adjusted to produce the correct mixture.

The invention may be illustrated by the following example.

A standard'compressed gas cylinder was used having a nominal compressed gas capacity of 220 cu. ft. at 132 atmospheres pressure. A weighed amount of nitrous oxide was charged into the cylinder, following which oxygen was introduced until the pressure reached 135 atmospheres. The composition of the mixture was found on analysis to be 74.9 of nitrous oxide and 25.1% of oxygen. The cylinder was allowed to discharge, and at intervals samples of the issuing gas were taken from the cylinder in its normal upright position and in the inverted position. The nitrous oxide content of the samples was determined by measurement of the difference between the refractive index of the gas mixture and that of oxygen. The results obtained which are subject to an error of i0.2% were as follows: 1

I Nitrous oxide Cylinder press, p.s.i. Cylinder position Tempeature, percerit by 1,700 Upright 21.1 74.7 Inverted 21. 1 74. 7

1,500 Upright 23.0 74. 9 Inverted- 23. 0 74. 7

1,300 Upright 23. 8 75.0 Inverted 23. 8 75. 0

1, 100 Upright 20. 2 74. 6 Inverted 20. 2 74. 6

900. Unri ht 22. 2 74. 7 Inverted 22. 2 74. 7

500 Upright 23. 0 74. 7 Inverted. 23. 0 74. 7

300 Upright 2A. 8 75. 2 Inverted 24. 8 75. 2

100 Upright 27. 2 75. 2 Inverted V 27. 2 75.2

i Percent volume of nitrous oxide in oxygen Pressure, p.s.1.g.

2. 504 Unsafe (eyl. full of liquid). 1. 300 2. 225 2. 476 Do. 1. 000 1. 665 1. 925 Do 0. 731 1.145 1. 247 Do 0. 469 0. 678 0. 696 o 0.357 0. 485 r 0. 485 2.667. 0. 244 0. 308 0. 308

The following is illustrative of the case where a more potent drug, halothane, is included as an adjuvant in a nitrous oxide/oxygen mixture.

A cylinder of 538 gms. water capacity was evacuated, and 3.75 ml. halothane (7.01 g.) was admitted. The

4 cylinder was then charged to a pressure of 1,620 p.s.i.g. at C. with a mixture of 68.51% volume nitrous-oxide and 31.49% volume oxygen. Samples withdrawn from the cylinder as it was progressively emptied, gave the halothane content indicated below:

Cylinder con- Cylinder Halothane tents in litres pressure, volume, per- N.T.P. p.s.i.g., 20 C. cent v./v.

The small differences-in halothane content were caused by sampling difiiculties. They are not significant and show that the halothane content remained substantially constant.

Mixtures of nitrous oxide with-permanent gases other than oxygen can be prepared in a manner. similar to that described above to yield a mixture of constant composition, evenif the partial, pressure of nitrous oxide isin excess of 50 ats. A specific example of such amixture is a nitrous oxide/nitrogen mixture containing up to 75% nitrous oxide in a pressure container to 132 ats. Such mixtures are of use in leak detection, using infra red apparatus where it is necessary for the test procedure to be conducted at relatively high pressure. Such high pressure testing is sometimes necessary for detecting leaks in water mains and other hydraulic equipment such as tubular condensers.

We claim;

1. The method of forming a homogeneous mixture of a permanent gas .and nitrous oxide, which comprises charging the nitrous oxide into a valved pressure container, subsequently charging into the same container a permanent gas so that the subsequent mixture includes from 15 up to 75% by volume of nitrous oxide,and until a final pressure is reached at which the partial'pressure of nitrous oxide is greater than 50 ats.

2. The method according to claim 1 in whichthe contents of the pressure container are. agitated to accelerate the attainment of equilibrium.

3. The method according to claim 1 in which the pressure container is inverted after being charged with nitrous oxide, and the permanent gas is bubbled through the liquid nitrous oxide until equilibrium is attained.

4. The method according to claim 1,;in which the permanent gas is added intermittently to allow equilibrium to be attained.

5. The method according to claim 4, in which the contents of the pressure container are agitated to accelerate the attainment of equilibrium. 7

6. The method of forming a homogeneousmixture of a permanent gas and of nitrous oxide, which comprises charging the gases in proportions of from 15% up to 75% by volume of the nitrous oxide into a pressure container until a final pressure is reached at which the partial pressure of nitrous oxide is greater than 50 ats.

7. The method of forming a homogeneous mixture of oxygen and nitrous oxide, which comprises charging the gases in proportions of from 15% up to 75% volume of the nitrous oxide into a pressure container until a final pressure is reached at which the partial pressure of nitrous oxide is-greater than 50 ats.

8. The method according to claim 7, in which an adjuvant is included in the mixture.

9. The method according to claim 7, in which from a trace up to 1% by volumeof halothane is included in the mixture as an adjuvant.

10. The method according to claim '7, in which from a trace up to by volume of cyclopropane is included in the mixture as an adjuvant.

11 As an article of manfuacture, a pressurized dispensing container Within which is a homogeneous gaseous medium comprising from 15% up to 75% by volume of nitrous oxide in admixture with a permanent gas, the partial pressure of nitrous oxide being greater than 50 ats.

12. As an article of manufacture, a pressurized dispensing container within which is a homogeneous gaseous medium comprising up to 75% by volume of nitrous oxide in admixture with oxygen, the partial pressure of nitrous oxide being greater than SO ats.

13. As an article of manufacture, a pressurized dispensing container Within which is a homogeneous gaseous medium comprising from 15% up to 75 by volume of nitrous oxide in admixture with oxygen and from a trace up 1% by volume of halothane, the partial pressure of nitrous oxide being greater than 50 ats.

14. As an article of manufacture, a pressurized dispensing container Within which is a homogeneous gaseous medium comprising from 15 up to 75% by volume of nitrous oxide in admixture with oxygen and from a trace up to 5% by volume of cyclopropane, the partial pressure of nitrous oxide being greater than 50 ats.

15. As an article of manufacture, a pressurized dispensing container within which is a homogeneous gaseous medium comprising from 15% up to 75% by volume of nitrous oxide in admixture with nitrogen, the partial pressure of nitrous oxide being greater than ats.

References Cited by the Examiner UNITED STATES PATENTS 8/61 Robson 16752.6

OTHER REFERENCES Brown, Surv. Anesth., vol. 4, No. 2, page 169, April 1960.

Johnstone, Surv. of Anesth., vol. 1, No; 3, page 181, June 1957.

Mush-in, British Medical Journal, No. 5084, June 14, 

12. AS AN ARTICLE OF MANUFACTURE, A PRESSURIZED DISPENSING CONTAINER WITHIN WHICH IS A HOMOGENEOUS GASEOUS MEDIUM COMPRISING UP TO 75% BY VOLUME OF NITROUS OXIDE IN ADMIXING WITH OXYGEN, THE PARTIAL PRESSURE OF NITROUS OXIDE BEING GREATER THAN 50 ATS. 